The present disclosure relates generally to implementing fallback to enable user equipment to obtain service from a network cell not currently associated with the user equipment, for example, circuit-switched fallback, and, more specifically, to minimizing delay and improving reliability for circuit-switched fallback.
As used herein, the term “device” can refer to a mobile station (MS), user agent (UA), or user equipment (UE), and can include electronic devices such as fixed and mobile telephones, personal digital assistants, handheld or laptop computers, smartphones, televisions and similar devices that have network communications capabilities. The terms may also refer to devices that have similar capabilities but that are not readily transportable, such as desktop computers, set-top boxes, IPTVs or network nodes. The term “UE” can also refer to any hardware or software component that can terminate a communication session that could include, but is not limited to, a Session Initiation Protocol (SIP) session. Also, the terms “user agent,” “UA,” “user equipment,” “UE,” and “node” might be used synonymously herein. Those skilled in the art will appreciate that these terms can be used interchangeably.
A UE may operate in a wireless communications network that provides high-speed data and/or voice communications. The wireless communications networks may implement circuit-switched (CS) and/or packet-switched (PS) communication protocols to provide various services. For example, the UE may operate in communications networks using different radio access technologies (RAT), such as an Enhanced Universal Terrestrial Radio Access Network (E-UTRAN), Universal Terrestrial Radio Access Network (UTRAN), Global System for Mobile Communications (GSM) network, Evolution-Data Optimized (EV-DO), 3GSM, Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN), Universal Mobile Telecommunications System (UMTS), Enhanced Data rates for GSM Evolution (EDGE), GPRS/EDGE Radio Access Network (GERAN), and/or General Packet Radio Service (GPRS) technology. Other wireless networks that UE may operate in include but are not limited to Code Division Multiple Access (CDMA), cdma2000, cdma2000 1xRTT, cdma2000 HRPD, WLAN (e.g. IEEE 802.11) and WRAN (e.g. IEEE 802.22). UE may also operate in fixed network environments such as example Digital Subscriber Line (xDSL) environments, Data Over Cable Service Interface Specification (DOCSIS) cable networks, Wireless Personal Area Networks (PAN), Bluetooth, ZigBee, Wireless Metropolitan Area Networks (MAN) (e.g., WiMAX, IEEE 802.20, IEEE 802.22 ethernet) or optical networks. Some UE may be capable of multimode operation where they can operate on more than one access network technology either on a single access network at a time or in some devices using multiple access technologies simultaneously.
In wireless telecommunications systems, transmission equipment in a base station transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. Such advanced equipment may include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB). Such advanced or next generation equipment may be referred to as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). As used herein, the term “access device” will refer to any component, such as a traditional base station, eNB, or other LTE access devices, that can provide UE with access to other components in a telecommunications system.
The different networks described above provide a variety of services to connected UE. Some networks, for example, provide only PS services and cannot provide CS voice or other CS domain services. As such, UE may be configured to connect to different types of networks to access both PS and CS domain services. For example, if UE is connected to a first network cell that does not provide CS domain service, the UE may be configured to implement CS fallback to connect to an accessible network such as a GERAN or UTRAN to access voice or other CS domain services provided by those networks. As such, a CS fallback procedure allows UE connected to a network using a first RAT and providing only PS domain services to connect to another network using a second RAT and providing CS domain services. CS fallback may be used, for example, to initiate voice calls via a cell of a network providing CS domain services, when, at the time of initiating the voice call, the UE was associated with a cell of a network that only provides PS domain services. The UE initiating the voice call may be either idle or active on the cell of the network that only provides PS domain services. In case the UE is idle it can be said to be camped on the cell and may be monitoring the paging channel of that cell for paging messages for mobile-terminated sessions or calls. In case the UE is active it may be communicating with the cell and transferring data for a PS domain service.
FIG. 1 is an illustration of a CS fallback process wherein UE 10 transitions from an E-UTRAN cell to a GERAN or UTRAN cell to access CS domain services for initiating a voice call. In FIG. 1, UE 10 is initially connected to E-UTRAN cell 100. Because E-UTRAN cell 100 does not provide CS domain services, UE 10 implements CS fallback to communicate with the GERAN or UTRAN cell 102 to access CS domain services. Depending upon network implementation, it is not necessary that cells 100 and 102 be co-extensive. However, to transfer from one cell to another, UE 10 should be within the communication range of each cell.
In FIG. 1, UE 10 is first connected to or camped on E-UTRAN cell 100. To initiate a mobile-originated voice call, UE 10 transmits a signal 104 to E-UTRAN cell 100 that includes a request to initiate a voice call. After receiving the request, E-UTRAN cell 100 transmits a signal 106 to UE 10 indicating that the voice call cannot be supported because E-UTRAN cell 100 cannot provide the necessary CS domain services. Signal 106 may also include a reference identifying a candidate GERAN or UTRAN cell 102, which does support the CS domain services for the voice call. After receiving signal 106, UE 10 transfers to GERAN or UTRAN cell 102 using the information provided in signal 106. The transfer may be implemented using a handover procedure, cell change order (CCO) procedure, PS handover procedure, or redirection procedure, for example. Note that in FIG. 1, arrow 108 only indicates the transfer of UE 10's communication from one cell to another, and does not indicate physical movement of UE 10. After transferring to GERAN or UTRAN cell 102, UE 10 establishes a connection for initiating the voice call as indicated by line 110. In the case of a mobile-terminated voice call, UE 10 may be first paged by E-UTRAN cell 100 for an incoming CS domain voice call. In response to the page, UE 10 follows a similar process as described above for the mobile-originated call to transfer to the GERAN or UTRAN cell 102 and after transferring UE 10 responds to the page on the GERAN or UTRAN cell 102.
To facilitate CS fallback, UE 10 may be configured to communicate with both PS-based and CS-based networks. For example, UE 10 may support combined procedures for EPS/International Mobile Subscriber Identity (IMSI) attach, and Tracking Area update for registering with a Mobility Management Entity (MME) to access PS domain services (for example, via an E-UTRAN, UTRAN or GERAN access network) and for registering with a Mobile Switching Center (MSC) to access CS domain services (for example, via a UTRAN or GERAN access network or another network supporting CS domain services). The combined procedures also allow the MSC and MME to create an association between one another so that each is aware that UE 10 is simultaneously registered with both the MSC and MME and that, therefore, the UE is registered with both the PS and CS networks.
When performing CS fallback, UE 10 may be in the best position to determine which cell or cells are candidate cells to fallback to—UE 10 can detect which cells are in close proximity or have particularly strong received signal strength or quality (or other such preferential parameters), and hence with which cells UE 10 would likely have a successful connection following the CS fallback process. As such, during the CS fallback process, UE 10 may undertake a measurement step to detect and identify the cells accessible to UE 10. In other words, before falling back to a cell providing CS domain services, UE 10 first searches for available candidate network cells via a measurement process.
The measurement step may involve interruptions in the downlink reception and uplink transmission activities of UE 10 during which UE 10's receiver is temporarily retuned to the frequencies that might be used by the candidate cells (e.g., in the case of GERAN candidate cells, the frequencies on which broadcast control channels (BCCH) may be transmitted) that may be accessible to UE 10. These interruptions are termed measurement gaps. The measurement gaps periodically occur. One standard currently defines 2 different periods: gap pattern 0, which gives a 6 ms measurement gap every 40 ms, and gap pattern 1, which gives a 6 ms measurement gap every 80 ms. Thus the measurement gap patterns give 7.5% (pattern 1) or 15% (pattern 0) of the UE 10's time to detect and perform measurements of cells of other networks, which therefore take a relatively long time.
FIG. 2 is an illustration of an exemplary measurement gap pattern that allows the receiver of UE 10 to be temporarily retuned to frequencies that might be used by GERAN cells to detect cells that are accessible to UE 10. At time t=0, RRC Connection Reconfiguration procedure is performed to begin the measurement process. Periodic measurement gaps 115 are then defined to allow UE 10 to perform measurements for GERAN cells. During measurement gaps 115, UE 10 reconfigures its receiver in an attempt to detect and/or measure available candidate cells. In the non-measurement gap periods 117, UE 10 assumes normal operation.
After the last measurement gap, sufficient measurements may have been performed by UE 10 and one or more candidate network cells that provide CS domain services may have been detected. UE 10 may then transmit measurement results to an access device of the PS network (e.g., an E-UTRAN eNB). The measurement results transmitted to the PS network may then be used by the PS network to determine an optimal CS network cell to which UE 10 may be transferred during the CS fallback procedure.
When implementing CS fallback, delay is a concern. If UE 10 is initially camped on an E-UTRAN cell and wishes to access CS domain services, a CS fallback process may be executed. While the RRC (radio resource control) connection setup procedure of the CS fallback process may be relatively short (150 ms is the target time for the E-UTRA system design) the measurement step and the step of selecting a target cell for CS domain services can potentially take a significant amount of time. As such, CS fallback may be delayed resulting in delays in establishing the CS domain services, possibly delaying the establishment of a voice connection for the user or negatively affecting other services accessed by UE 10. In particular, the need to carry out a number of steps while camped in E-UTRAN and possibly obtain system information for the target cell may result in a delay which, to the user, is noticeably longer than if UE 10 were initially camped on the target cell.
It is also possible that during CS fallback UE 10 may be directed to or select a target cell that has (or, in other words, is in, or belongs to) a different location area (LA) than the cell with which or in which UE 10 is currently registered. Any resulting location update may add further delay to establishing the CS domain service. Also, in a mobile-terminated call, it is possible that the LA of the target cell is associated with a different MSC from the MSC that handles the incoming call. In that case, call establishment may fail.